1. Field of the Invention
This invention relates to mobile and base station antennas and more particularly to a dual polarized tri-element (tri-band) antenna for use in vehicles. In a particular aspect, the antenna is used with a parabolic dish reflector and director elements to provide for high gain applications.
2. Description of Related Art
Currently there is a growing need for wireless mobile telephones. Common places on a vehicle for mounting mobile antennas include the roof, rain gutter, bumper, trunk lid, mirror bracket, fender and the side of the vehicle. The simplest mobile VHF/UHF antenna is the quarter wave vertical xe2x80x9cwhipxe2x80x9d antenna mounted on a high-grade standoff insulator on the roof of a car. The metal body of the vehicle serves as a ground plane but can distort the normal circular radiation pattern of a vertical antenna.
At the center frequency of the citizen""s band (27.185 MHz) a quarter wave antenna is 108.62 inches (about 10 feet). Such an antenna can strike many overhead obstructions, causing it to bend and alter the angle of radiation when the vehicle is moving. Mounting a 10-foot antenna on the roof of a car is not feasible. An antenna that is physically shorter than a quarter wavelength must have inserted into it a suitable loading coil to bring its electrical length up to a quarter wave.
The performance of a mobile whip antenna can be improved by adding capacitance to the portion of the antenna above the loading coil. This capacitance tends to resonate with the inductance of the coil. Since the impedance of the whip antenna is lower than that of the coaxial line that brings power from the transmitter, an impedance matching network is needed.
Additionally, in general, an antenna must be tuned to the same frequency band that the radio system to which it is connected operates in; otherwise, transmission and/or reception can be impaired. For the strongest signals, the transmitting and receiving signals each should have the same polarization, either horizontal or vertical. Oftentimes communication must be between stations that use vertical polarization and horizontal polarization. Reflections/refractions due to buildings/land masses cause cross-polarization of signals. Polarization of satellite signals is circular.
Franz U.S. Pat. No. 2,218,707 (issued October, 1940) discloses a dual polarization or dipole antenna for receiving and transmitting high frequency signals in conjunction with a substantially horizontal conducting vehicle panel defining a ground plane. In this antenna, three radiative elements are arranged vertically with two reflective radiators being mounted in vertical symmetrical relationship to a central radiator. The radiative elements do not have like ends secured in a common point or apex of a cone. Franz does not specifically teach specific frequency bands, or insulating the antenna elements from the vehicle ground, such as by a mounting block comprised of a dielectric material.
It is a general object of this invention to provide an improved multi-band antenna which is horizontally and vertically polarized and effective to transmit and receive in the broad frequency bands 140-170 MHz, 200-225 MHz and 400-480 MHz to include land mobile, HAM and satellite uses.
Another object of this invention is the provision of a mobile antenna that is compact and of low height, which permits mounting on the top of a vehicle.
Still another object of this invention is the provision of an antenna that does not require lossy band restrictive coils or windings (to bring the electrical length of the antenna up to a requisite wavelength) or tuning capacitors, thus providing an increase in signal strength.
Yet another object of this invention is the provision of a multi-element antenna system that significantly reduces flutter (picket-fencing) in the signal.
In another aspect of this invention, an object is the provision of a dual polarized antenna system used with a parabolic dish reflector to provide for high gain applications.
According to this latter aspect, an object is to associate an array of director rods, at the aperture plane of a parabolic dish reflector, to enhance the signal gain and directivity of signals sent to or received from a tri-element antenna positioned at the focal point below the aperture plane of the paraboloid.
In accordance with the present invention there is provided a dual polarization antenna for receiving and transmitting high frequency (VHF/UHF) signals in conjunction with a substantially horizontal conducting vehicle panel defining a ground plane, said dual polarization antenna comprising:
a first, second and third radiative element each comprised of an electrically conductive material, each said radiative element being generally linear and extending between a proximal end and a distal end, and
means mountable of the vehicle panel for securing the proximal ends together at a common point and in electrical circuit relation with one another for connection to a vehicle transceiver, the radiative elements extending vertically upwardly and outwardly from said common point whereby to form an imaginary cone with the proximal ends forming the apex of the cone,
said radiative elements each being of a different length and disposed at an angle relative to the ground plane to provide horizontal and vertical polarization and jointly resonate in a first, second and third frequency band.
In a preferred embodiment, the motor vehicle defines an electrical ground potential and the antenna is electrically insulated from said motor vehicle ground potential. The geometric axis of the cone is perpendicular to the ground plane and the radiative elements forming the cone are disposed at an angle of about 20xc2x0 to 45xc2x0 relative to the geometric axis of the cone (i.e., the cone has an angle of about 40xc2x0 to 90xc2x0 and is symmetrically aligned with the geometric axis). The radiative elements have a length of about 16 and 19 inches and the distal ends, if equal-lengthed, are circumferentially spaced at 120xc2x0 relative to the base of the imaginary cone.
Preferably, the radiative elements forming the cone are disposed at an angle of about 60xc2x0 to the ground plane and resonate in the frequency bands of about 140-170 MHz, 200-225 MHz and 400-480 MHz; and the length of the first, second and third radiative element is, respectively, about 16 xc2xe inches, 18 xc2xd inches and 19 inches. In another preferred embodiment, the elements of the cone have respective lengths of about 16 inches, 17 xc2xe inches, and 18xc2xc inches long.
Advantageously, an antenna having the above construction eliminates xe2x80x9clossyxe2x80x9d coils, capacitors and matching structures and has high power handling capabilities (200+ watts); achieves transceiving efficiency/gain in multiple frequency bands; provides broad frequency in each frequency band; reduces null/flutter problems; provides effective xe2x80x9cdualxe2x80x9d polarization radiation away from the horizon in addition to efficient xe2x80x9cnearxe2x80x9d horizon pattern; and provides extremely wide efficient continuous frequency receiving capabilities in a simple but compact construction.
According to another aspect of this invention is provided a dual polarization antenna for receiving and transmitting high frequency signals in conjunction with a substantially horizontal conducting panel defining a ground plane. According to this aspect, the dual polarization antenna comprises:
an electrically conductive first, second and third antenna element to receive horizontally and vertically polarized components and jointly resonate within three separate frequency bands, each said antenna element being axially elongated and having a first end and a second end, and
a mounting block comprised of a dielectric material adapted to be mounted to the vehicle panel, said mounting block electrically insulating said first, second and third antenna elements from the vehicle ground plane and securing said antenna elements into a triangular arrangement such that said second ends are in electrical circuit path relation with one another and the first ends are spaced vertically upwardly and above said mounting block and circumferentially spaced at 120xc2x0, the antenna elements forming an imaginary cone having a center geometrical axis that is generally perpendicular to the ground plane of the vehicle.
Preferably, the antenna elements are disposed at predetermined angle relative to the ground plane to provide horizontal and vertical polarization in the first, second, and third frequency bands.
According to yet another important aspect of this invention, an antenna comprises three radiative elements for transmitting and receiving electromagnetic radiation within predetermined frequency bands, the radiative elements being electrically connected at common point at one end of a radiative member and forming an imaginary cone. Further, this antenna comprises:
a truncated parabolic reflector dish, said reflector dish having a parabolic surface portion conforming to a paraboloid of revolution about a central geometric axis, a focal point on the axis, a rearward end wall, and a forward end portion, and
first means for mounting the radiative member on said parabolic dish and positioning said radiative elements relative to said axis and substantially at said focal point.
The reflector dish has an opening defined by a diameter xe2x80x9cdxe2x80x9d and the focal point xe2x80x9cfxe2x80x9d of the parabolic dish is located on the central geometric axis of the parabola. The reflector dish may be shallow or deep, depending on the application. Preferably, the focal point is between the vertex of the parabola and the opening of the dish. Preferably, the antenna has a ratio of xe2x80x9cf/dxe2x80x9d of about 0.01 to about 0.625. More preferably, the ratio of xe2x80x9cf/dxe2x80x9d is about 0.21.
Inasmuch as the radiative elements are axially elongated, the elements are not located, as a point source, at the focal point of the parabola. Preferably, the midpoint of the axially elongated radiative elements is located substantially at the focal point of the reflective dish. Further, the radiative elements may be of the same or different length.
In an additional preferred aspect, the antenna further comprises: second means for supporting and positioning a plurality of conductive director rods in spaced overlying relation to said radiative member, said director rods being substantially coplanar and disposed in a plane orthogonal to said axis and spaced away from said end wall and said focal point.
In this regard, the second means preferably comprises a generally planar support member that is connected to the opening of said reflector dish, and the director rods are connected to said support member and positioned above the radiative elements. Preferably, the support member is comprised of a material substantially transparent to electromagnetic energy.
In this additional preferred aspect, the director rods are disposed on a radius extending outwardly from the axis of the reflector dish and arranged in a first and second sets. The first set of rods forms the spokes of a wagon wheel (or a xe2x80x9cstarxe2x80x9d) that is centered on the axis, each of the rods (spokes) of the first set being generally at 60xc2x0 to one another. The second set of rods is disposed within an annular band that encircles the wagon-wheel, the annular band being centered on the axis and the director rods being generally at 60xc2x0 to one another. The director rods of the first and second sets are angularly offset at about 30xc2x0 to one another.
The novel features of this invention are set forth with particularity in the appended claims. The invention itself will be best understood from the following description when read in connection with the accompanying drawings.